This invention relates to copolymers and, more particularly, to chemical and solvent resistant polyimides containing 3,3',4,4'-tetracarboxybiphenyl dianhydride (BPDA) moieties.
Aromatic polyimides and polyamide-imides have found extensive use in industry as fibers, composites, molded parts and dielectrics (insulators or non-conductors) due to their toughness, flexibility, mechanical strength and high thermal stability. In the electronic industry, polyimide coatings are widely used as dielectrics in the fabrication of microelectronic devices. The advantages of using polyimides over inorganic dielectrics such as silicon dioxide or silicon nitride, are a low dielectric constant, ease of application, crack resistance, and a tendency to planarize substrate topography better than most inorganic materials.
Dielectrics typically are coated with thin films of a patterned conducting material. As such, they are used to provide an electrical network to interconnect the individual semiconductor devices of an integrated circuit. These electrical networks provide electrical power to, and transfer electrical signals between, individual semiconductor devices. In other words, the dielectrics can be used as a component in a system that provides an electrical interconnection network for an individual semiconductor device which is composed of integrated circuits built on a common substrate. The dielectrics can also be used to form interconnection networks of integrated circuits as part of a multi-chip module or multi-package module.
Essentially, the layers of the interconnection network are layers of patterned conducting material which are separated from the substrate and from each other by layers of dielectric, typically a non-conducting polymer. The conducting layer is etched to form a pattern of current carrying conducting material, typically referred to as conducting lines or traces, over the surface of the underlying dielectric layer. Thereafter, a subsequent layer of dielectric is applied to cover both the trace and the uncovered portions of the underlying dielectric layer. In practice, the subsequent layer of polymer is applied over a topographical (i.e., nonplanar) surface. Where the dielectric is a polymer, the polymer tends to planarize the substrate topography. However, some topography will remain and there will be some areas where the polymer is thinner than other areas. For example, the polymer layer over the trace will be thinner than the polymer layer over the uncovered portions of the underlying dielectric layer.
If the polymer thickness is further reduced by the chemicals or solvents used in subsequent process steps, the thinner regions of polymer coating can be removed to the extent that the conducting material is exposed through the polymer coating. Such exposed conducting material can cause short circuits between conductors. In addition, extremely thin areas or layers of polymer coating can lead to large electric fields which can contribute to the breakdown of the insulating properties of the dielectric.
In addition, two key electrical characteristics, the network capacitance and the speed of signal propagation, are controlled by the dielectric coating thickness. The network capacitance determines the amount of electric charge the semiconductor device must supply to the network in order to transmit a signal. The electronic components are designed to handle a predetermined capacitance and when the polymer coating thickness decreases the capacitance goes up such that it can increase beyond the capability of the electronic components to handle that increase. As a result, the electronic components will not be able to propagate a signal.
Polymer coating thickness can also affect the speed of signal propagation. The integrated circuit is designed to operate at a particular speed of signal propagation and, if signals do not propagate as required, the performance of the integrated circuit will be reduced. In this case, uneven layers of dielectric will impair the electrical performance of the semiconductor device.
Thus, the dielectric coating thickness must be controlled both during the dielectric coating deposition process and during subsequent semiconductor device fabrication processing steps. When the dielectric is a polyimide, the fabrication processing steps, for example, in the case of a typical interconnect network fabrication process, requires a multistep procedure. In this case, the polyimide must be resistant to the chemicals and solvents used in the many steps of the fabrication process.
Accordingly, it is an object of the present invention to provide an improved polyimide composition for microelectronic applications. Another object of the present invention is to provide a polyimide composition which is resistant to the chemicals and solvents used in the many steps of the interconnect fabrication process. A still further object of the present invention is to provide a polyimide composition having advantageous properties. These and other objects and advantages of the present invention will be apparent to those skilled in the art from the following description.